Membrane and electrode structure

ABSTRACT

The electrode layer of a ion exchange membrane is formed by printing a ink of catalytically active particles on the surface of the membrane. The inventive electrode ink comprises: 
     (a) catalytically active particles; 
     (b) a hydrocarbon having at least one ether, epoxy or ketone linkage and an alcohol group, preferably 1-methoxy 2-propanol; and 
     (c) optionally a binder, preferably perfluorinated sulfonyl fluoride polymer or perfluorinated sulfonic acid polymer.

FIELD OF THE INVENTION

This invention relates to a membrane and electrode structure composed ofan ion exchange membrane having a plurality of electrically conductive,catalytically active particles present on one, or both, surfaces of anion exchange membrane. The electrically conductive, catalytically activeparticles serve as a particulate electrode when the membrane andelectrode structure are used in an electrochemical cell. Membrane andelectrode structures are sometimes called solid polymer electrolytestructures or SPE structures. The membrane and electrode structures areparticularly useful in fuel cells.

BACKGROUND OF THE INVENTION

So-called "M & E cells" are electrochemical cells employing a membraneand electrode structure. Such cells can be operated as an electrolyticcell for the production of electrochemical products, or they may beoperated as fuel cells for the production of electrical energy, gasgenerating devices and processes, chemical synthesis devices, chemicaltreatment and processing devices and methods, gas dosimeters and sensingdevices and the like. Electrolytic cells may, for example, be used forthe electrolysis of an alkali metal halide such as sodium chloride orfor the electrolysis of water. M & E cells are well known in the art.

The contact of the gas-liquid permeable porous electrode with the ionexchange membrane is an important factor for the efficiency of the M & Ecell. When the thickness of an electrode is nonuniform or the contactbetween the electrode with the ion exchange membrane is notsatisfactory, a part of the electrode is easily peeled off adverselyeffecting the electrical properties of the cell. The advantages of the M& E structure are then decreased or lost.

Membrane and electrode structures are currently manufactured by severaltechniques. U.S. Pat. No. 3,297,484 illustrates in detail materials forelectrode structures including exemplary catalyst materials forelectrodes, ion exchange resins for membrane and electrode structuresand current collecting terminals. Catalytically active electrodes areprepared from finely-divided metal powders, customarily mixed with abinder such as polytetrafluoroethylene resin. The electrode is formedfrom a mixture of resin and metal bonded upon one or both of thesurfaces of a solid polymer matrix, sheet or membrane.

In U.S. Pat. No. 3,297,484, the mixture of resin and catalyticallyactive particles is formed into an electrode structure by forming a filmfrom an emulsion of the material, or alternatively, the mixture of resinbinder and catalytically active particles is mixed dry and shaped,pressed and sintered into a sheet which can be shaped or cut to be usedas the electrode. The mixture of resin and catalytically activeparticles may also be calendered, pressed, cast or otherwise formed intoa sheet, or fibrous cloth or mat may be impregnated and surface coatedwith the mixture. In U.S. Pat. No. 3,297,484, the described electrodesare used in fuel cells. In U.S. Pat. No. 4,039,409, the bonded electrodestructure made from a blend of catalyst and binder is used as theelectrode in a gas generation apparatus and process.

In U.S. Pat. No. 3,134,697, many ways are described for incorporatingcatalytically active electrodes into the surfaces of an ion exchangemembrane. In one embodiment, as explained above, the electrode materialmade of catalytically active particles and a resin binder may be spreadon the surface of an ion exchange membrane or on the press platens usedto press the electrode material into the surface of the ion exchangemembrane, and the assembly of the ion exchange membrane and theelectrode or electrode materials is placed between the platens andsubjected to sufficient pressure, preferably at an elevated temperature,sufficient to cause the resin in either the membrane or in admixturewith the electrode material either to complete the polymerization if theresin is only partially polymerized, or to flow if the resin contains athermoplastic binder.

It is known to add binders, such as fluorocarbon polymers includingpolytetrafluoroethylene and polyhexylfluoroethylene, to the electrodeink. It is also known to add viscosity regulating agents such as solubleviscous materials to the electrode ink.

A method to construct membrane and electrode structures is alsodescribed in "Methods to Advance Technology of Proton Exchange MembraneFuel Cells;" E. A. Ticianelli, C. Derouin, A. Redondo and S. Srinivasanpresented at Second Symposium "Electrode Materials and Processes forEnergy Conversion and Storage," 171st Electrochemical Society Meeting,May, 1987. In this approach, a dispersion of a flocculent precipitate of20% platinum on a catalyst and TEFLON® (commercially available from E.I. du Pont de Nemours and Company) is prepared. The flocced mixture iscast onto paper and then pressed onto a carbon paper substrate. Theelectrodes may then be sintered at elevated temperature, approximately185° C., for 30 minutes. The electrode is next brushed with a solutionof chloroplatinic acid and subsequently reduced with an aqueous mixtureof sodium borohydride.

The electrode is then washed and NAFION® (commercially available from E.I. du Pont de Nemours and Company) solution brushed on the surface ofthe electrode. The method of solution processing is described in"Procedure for Preparing Solution Cast Perfluorosulfonate Ionomer Filmsand Membranes," R. B. Moore and C. R. Martin, Anal. Chem., 58, 2569(1986), and in "Ion Exchange Selectivity of NAFION® Films on ElectrodeSurfaces," M. N. Szentirmay and C. R. Martin, Anal. Chem., 56, 1898(1984). The so-called NAFION® solution may be made from a solvent whichis, for example, a lower-boiling alcohol such as propanol or ahigh-boiling alcohol such as ethylene glycol. In the case of thehigher-boiling alcohol, the treated electrode is heated to about 140° C.in an inert gas to drive off the alcohol. The electrodes are then washedin hot hydrogen peroxide solution and then in nitric acid. This NAFION®impregnation step is followed by hot pressing the electrodes onto an ionexchange membrane for a sufficient time at suitable temperatures andpressures.

Using transfer catalyzation wherein an electrode ink comprising aplatinum catalyst on a carbon supporting material is printed on asuitable substrate, such as TEFLON® or paper, it has been possible toform electrodes containing as little as 0.2 mgm/cm² of precious metal.In particular, these electrodes, which are essentially decals formedfrom a supported platinum catalyst electrode ink are painted or sprayedon the substrate and then dried and hot pressed onto ion exchangemembranes. This so-called decal process of applying the electrode ink tothe surface of the membrane has been successful but involves the arduousprocess steps of forming the electrode decal and then transferring it tothe membrane.

In all of the foregoing techniques, it has been necessary to utilizeliquid-based emulsion and several processing steps to form film of theelectrode material and thereafter bind or press the sheet of electrodematerial upon the ion exchange membrane, or it has been necessary to usebinders and substantial quantities of expensive catalyst materials toprepare membrane and electrode structures. It has also been necessary toutilize large loadings of catalyst to make acceptable electrodes inthese prior art methods. The process for preparing the electrodes usingprior art ink compositions is inefficient and the reproducibility ispoor.

By prior art techniques, it has been impossible to prepare membrane andelectrode structures having loadings of the unsupported catalystmaterials as low as 3.0 mg per cm² or even lower with no compromise inthe integrity of the membrane or the performance of the membrane andelectrode structure in various fuel cells, gas generating systems andother devices.

U.S. Pat. No. 4,272,353 tries to solve some of these problems byabrading or physically roughening the surface of the membrane to providea support for locking, uniting or fixing the finely-divided catalystparticles to the surface of the membrane. Particularly, before thecatalyst is deposited upon the surface of the membrane, the surface issubjected to a suitable abrading or roughening means. However, theabrasion process can result in deleterious effects to the strength,dimensional stability and electrical properties of the membrane.Moreover, abrading the membrane requires an additional process step.

Moreover, directly applying catalyst inks to a membrane which is in theproton form has been largely unsuccessful. The alcohol carrier causesswelling and distortion of the membrane onto which it is applied.

It is also known to incorporate additives into the ink composition inorder to form a suspension of the catalytically active particles and/orbinder agents. Additives such as tetrabutyl ammonium hydroxide glycerolsand ethylene glycol are known additives which facilitate the printing ofthe electrode ink onto the surface of the membrane, but such additivesadversely interact with many binders and the ion exchange polymerscontained in the membrane.

Therefore, an electrode ink is needed which may be efficiently,inexpensively, and reproducibly applied to an ion exchange membrane, soas to form a uniform electrode structure which uses a relatively smallloading of catalyst does not crack or deform during operation, does notadversely decrease ionic conductivity of the structure, does not effectthe strength of the structure and does not adversely interact with theion exchange polymer contained in the membrane.

SUMMARY OF THE INVENTION

The present invention is an electrode ink which may be used to form amembrane and electrode structure having excellent characteristics. Theink comprises:

a) catalytically active particles (supported or unsupported), preferably5-40% by weight;

b) a suspension medium comprising a hydrocarbon having an ether, epoxyor ketone linkage and an alcohol group, which is preferably nonsolid atprocessing temperatures, preferably 50-95% by weight, such suspensionmedium preferably being 1-methoxy 2-propanol ("MOP");

c) binders such as perfluorinated sulfonyl fluoride polymer, preferably0-25% by weight, such polymer preferably being NAFION® perfluorinatedsulfonyl fluoride polymer (commercially available from E I. du Pont deNemours and Company), preferably in a solution of hydrocarbon solvent,or perfluorinated sulfonic acid polymer, preferably 0-25% by weight,such polymer preferably being NAFION® perfluorinated sulfonic acid(commercially available from E. I. du Pont de Nemours and Company),preferably in a solution of alcohol such as propanol or isopropylalcohol and water.

The electrode ink is printed, coated or bonded onto the surface of themembrane by methods known in the art. The ink readily adheres to themembrane thereby reducing the likelihood of delamination of theelectrode structure, uniform application of the electrode layer,reduction in the formation of gas bubbles at the membrane/electrodeinterface and without adversely effecting the strength, dimensionalstability or electrical properties of the membrane. Unlike prior artmembranes the suspension medium reduces the viscosity of the ink,suspends or dissolves the polymer but does not interact with thefunctional groups of the polymer which may reduce the ionic conductivityof the membrane and electrode structure.

The inventive membrane and electrode structure is particularly useful infuel cells.

DETAILED DESCRIPTION OF THE INVENTION

The electrode ink of the present invention comprises:

a) catalytically active particles (supported or unsupported), preferably5-40% by weight;

b) a suspension medium comprising a hydrocarbon having an ether, epoxyor ketone linkage and an alcohol group, which is preferably nonsolid atprocessing temperatures, preferably 50-95% by weight, such suspensionmedium preferably being 1-methoxy, 2-propanol ("MOP");

c) binders such as perfluorinated sulfonyl fluoride polymer, preferably0-25% by weight, such polymer preferably being NAFION® perfluorinatedsulfonyl fluoride polymer (commercially available from E I. du Pont deNemours and Company), preferably in a solution of hydrocarbon solvent,or perfluorinated sulfonic acid polymer, preferably 0-25% by weight,such polymer preferably being NAFION® perfluorinated sulfonic acid(commercially available from E. I. du Pont de Nemours and Company),preferably in a solution of alcohol such as propanol or isopropylalcohol and water.

The electrode layer can be made from well-known catalytically activeparticles or materials. The anode is preferably formed by one or moreplatinum group metal such as platinum, ruthenium, rhodium, and iridiumandelectroconductive oxides thereof, and electroconductive reducedoxides thereof. The cathode is preferably formed by one or more of iron,nickel, stainless steel, a thermally decomposed product of a fatty acidnickel salt, Raney nickel, stabilized Raney nickel, carbonyl nickel andcarbon powder supporting a platinum group metal. The catalyst may besupported orunsupported. The preferred catalyst is a platinum catalyst(manufactured byPrecious Metals Corp.), particularly 20% platinum on acarbon support knownas VULCAN® (manufactured by Cabot Corp.).

The catalytically active material is conventionally incorporated in theinkin a form of a powder having a particle diameter of 100 Angstroms to1000 Angstroms, especially 120 Angstroms to 500 Angstroms.

A hydrolyzed or unhydrolyzed sulfonyl fluoride polymer, preferably apolymer solution, is incorporated in the ink. The polymer is typicallyused as a binder for the electrode and the ion exchange membrane. Thepolymer facilitates the bond between the electrode ink and the surfaceof the membrane without significantly impairing or reducing the ionicconductivity of the membrane and electrode structure.

The suspension medium is a hydrocarbon having an ether, epoxy or ketonelinkage and an alcohol group, which is nonsolid at processingtemperatures. The preferred suspension medium is MOP. Other suitablesuspension media include 1-ethoxy-2-propanol; 1-methoxy 2-methyl2-propanol; 1-isopropoxy 2-propanol; 1-propoxy 2-propanol; 2-phenoxy1-propanol; 2-ethoxy 1-propanol; 2,3-ethoxy 1-propanol; 2-methoxy1-propanol; 1-butoxy 2-propanol; or mixtures thereof. In the foregoingexamples, the propanol constituent may be substituted with otheralcohols,for example, ethanol or butanol.

The suspension media of the present invention are particularly usefulbecause they act as a solvent, carrier or suspension agent for thecatalytically active particles and the perfluorosulfonic acid polymer(or the perfluorinated sulfonyl fluoride polymer). Moreover, thesuspension media do not significantly interact with the functionalgroups of the perfluorosulfonic acid polymer (or the perfluorinatedsulfonyl fluoride polymer) which could impair or reduce the ionicconductivity of the membrane and electrode structure during operation.In addition, the suspension media act as a viscosity regulating whichfacilitates the printing or coating of the electrode ink on the surfaceof the membrane, without interacting with the ion exchange polymerscontained in the membrane.

Binders are well known in the art. The preferred binder of the presentinvention is a perfluorinated sulfonyl fluoride polymer. The sulfonylpolymers (and the corresponding perfluorinated sulfonic acid polymers)with which the present invention is concerned are fluorinated polymerswith side chains containing the group --CF₂ CFR_(f) SO₂ X, wherein R_(f)is F, Cl, CF₂ Cl or a C₁ to C₁₀ perfluoroalkyl radical, and X is F orCl, preferably F. Ordinarily, the side chains will contain --OCF₂ CF₂CF₂ SO₂ X or --OCF₂ CF₂ SO₂ F groups, preferably the latter. For use inchloralkali membranes, perfluorinated polymers are preferred. Polymerscontaining the side chain --OCF₂ CF{CF₃ }O)_(k) --(CF₂)_(j) --SO₂ F,where k is 0 or 1 and j is 2, 3, 4, or 5,may be used. Polymers maycontain the side chain --CF₂ CF₂ SO₂ X where X is F or Cl, preferably F.

Preferred polymers contain the side chain --(OCF₂ CFY)_(r) --OCF₂CFR_(f) SO₂ X, where R_(f), Y and X are defined aboveand r is 1, 2, or3. Especially preferred are copolymers containing the side chain --OCF₂CF{CF₃ }OCF₂ CF₂ SO₂ F. Other suitable binders include fluorocarbonpolymers such as polytetrafluoroethylene and polyhexylfluoroethylene. Inorder to improve the dispersibility, it is possible to incorporate along chain hydrocarbontype surfactant or a fluorinated hydrocarbon typesurfactant at a desired ratio.

The preferred contents of the catalytically active particles and thebinderin the ink are generally dependant upon characteristics of theelectrode. In the case of fuel cell electrodes, the preferred ratio ofion exchange polymer to carbon support weight of the catalyst is in theratio of about 1:3.

The viscosity of the ink comprising the electrode powder is preferablycontrolled in a range of 1 to 10² poises especially about 10² poisesbefore printing. The viscosity can be controlled by (i) selectingparticle sizes, (ii) composition of the catalytically active particlesandbinder, (iii) a content of water as the medium or (iv) preferably byincorporating a viscosity regulating agent.

Suitable viscosity regulating agents include cellulose type materialssuch as carboxymethyl cellulose, methyl cellulose, hydroxyethylcellulose, and cellulose and polyethyleneglycol, polyvinyl alcohol,polyvinyl pyrrolidone, sodium polyacrylate and polymethyl vinyl ether.

The amount of catalyst material which is contained in the ink and whichdeposited upon the surface of the membrane in accordance with theprocess of the present invention is not critical. In a publicationentitled "Pseudohomogeneous Catalyst Layer Model for Polymer ElectrolyteFuel Cell," T. Springer and S. Gottesfeld, Los Alamos NationalLaboratory, Modeling of Batteries and Fuel Cells, ElectrochemicalSociety, PV91-10, 1991, it was shown that fuel cell electrode thicknessshould be constructed to be about 5 microns thick. This thicknessprovides a balancebetween proton conductivity and oxygen permeability inthe polymer of the catalyst layer. It has been found in accordance withthe present inventionthat the ink of the present invention permits thedeposition of surprisingly small quantities of catalyst material uponthe surface of themembrane. This value includes the weight of theprecious metal catalyst andexcludes the support. In accordance with thepresent invention, catalyst particles may be deposited upon the surfaceof a membrane in a range from about 0.2 mg. catalyst/cm² (supported) upto about 20 mg/cm² (unsupported) and higher. However, at higherloadings, that is loadings ofcatalyst over about 2.0 mg/cm², it may bemore important to add a binder to cause better adhesion or fixing of thecatalyst material upon the surface. However, binders are optional andare not required for structural integrity at loadings of catalyst ofabout 2.0 mg/cm² or less.

Catalyst is added to the surface of the membrane in an ink or ink form.Thearea of the membrane, which may be the entire area or only a selectportionof the surface of the membrane, is covered with the catalyticmaterial. Theexact amount of catalyst may be placed upon the surface ofthe membrane, that is, the desired loading. If necessary, appropriatesteps may be takento remove the excess catalyst material, such as byvibration, electrostatics, shaking, pouring, brushing, vacuum, and thelike. The catalyst ink may be deposited upon the surface of the membraneby any suitable technique including spreading it with a knife or blade,brushing,pouring, dusting, electrostatics, vibrating and the like. Areasupon the surface of the membrane which require no catalyst material, canbe masked,or other means can be taken to prevent the deposition of thecatalyst material upon such areas. The desired loading of catalyst uponthe membrane can be predetermined, and the specific amount of catalystmaterial can be deposited upon the surface of the membrane so that noexcess catalyst is required. For example, if 0.25 mg/cm² catalyst isdesired upon the surface of the membrane, that specific amount ofcatalystmaterial can be deposited upon the surface and fixed thereon. Inthis manner, any waste of relatively expensive catalyst materials can beavoided.

There are a number of suitable ways for depositing the particles ontothe membrane. For example, one can form a slurry of the catalyticallyactive particles and paint or spray the slurry onto the membrane.Spraying the solution/dispersion onto the flat electrically conductivescreen is used to advantage for covering large or irregular shapes.Pouring the solution/dispersion onto the membrane is sometimes used.Painting the solution/dispersion with brush or roller has beensuccessfully employed. In addition, coatings may be easily applied withmetering bars, knives, orrods. Usually, the coatings or films are builtup to the thickness desired by repetitive application.

A particular advantageous method of applying the catalytic particles tothemembrane is to blend the ink which is to be imprinted on the surfaceof themembrane. The ink is printed on and bonded to the surface of theion exchange membrane by the screen printing process. The conventionalscreen printing process can be employed. It is preferable to use ascreen having mesh number of 10 to 2400 especially mesh number of 50 to1000 and a thickness of 1 mil to 100 mils, especially 5 mils to 15 mils.When the mesh number is too large, the clogging of the screen results innonuniformprinting. When the mesh number is too small, excess of the inkis printed. When the thickness is too thick, too heavy a coating iscaused. When the thickness is too thin, a printing for a desired amountof the ink is not attained. A screen mask is used for forming anelectrode layer having a desired size and configuration on the surfaceof the ion exchange membrane. The configuration is preferably a printedpattern eliminating the configuration of the electrode. The thickness ofscreen mask is preferably in a range of 1 to 500 mu. The substances forthe screen and the screen mask can be any materials having satisfactorystrength such as stainless steel, polyethyleneterephthalate and nylonfor the screen and epoxy resins for the screen mask.

A screen and the screen mask are placed on the ion exchange membrane forthe printing of the electrode layer. The ink is fed on the screen and isprinted under a desired pressure by squeegee whereby the electrode layerhaving the configuration beside the screen mask, is formed on thesurface of the membrane. The thickness of the electrode layer on themembrane depends on the thickness of the screen, the viscosity of theink and the mesh number of the screen. It is preferable to control thethickness of the screen, the viscosity of the ink and the mesh of thescreen so as to give the thickness of the electrode ranging from 1micron to 50 microns, especially 5 microns to 15 microns.

The gap between the screen and the membrane, the material of thesqueegee and the pressure applied to mesh by the squeegee in the screenprinting process, highly relate to the physical properties, thicknessand uniformity of the electrode layer to be formed on the surface of themembrane. In order to give desired printing, the gap between the screenand the membrane is set depending upon the kind and viscosity of the inkpreferably ranging from 0.5 mm to 5 cm. The hardness of the squeegee isselected according to the viscosity of the ink, preferably ranging from50to 100 shore hardness. Preferably, uniform pressure of the squeegee isapplied to the mesh. Thus, the electrode layer having uniform thicknessisformed on one or both of the surfaces of the membrane in a highbonding strength. Thereafter, it is preferable to warm the electrodelayer to about 50° C. to 140° C., preferably about 75° C. The electrodelayer may be warmed by a lamp, usually about one foot away from themembrane or by other conventional means. This screen printing processmay be repeated until the desired loading of ink is achieved. Two tofour passes, usually three passes, produce the optimum performance.

Thereafter, it is preferable to fix the ink on the surface of themembrane.The ink may be fixed upon the surface of the membrane by anyone or a combination of pressure, heat, adhesive, binder, solvent,electrostatic, and the like. The preferred embodiment for fixing the inkupon the surfaceof the membrane are by pressure, by heat or by acombination of pressure and heat. Pressure and heat may be adjusted byone skilled in the art. It is preferable to press the electrode layer onthe surface of the membrane at 100° C. to 300° C., preferably 150° C. to280° C., most preferably 130° C. under a pressure of 510 to 51,000 kPa(5 to 500 atm) preferably 1015 to 101,500 kPa (10 to 100 atm), mostpreferably 2030 kPa (20 atm) whereby a strongly bonded structure of theelectrode layer and the ion exchange membrane can be obtained.

The electrode layer formed on the membrane should preferably be a gaspermeable porous layer. The average pore diameter is in a range of 0.01to50 mμ, preferably 0.1 to 30 mμ. The porosity is generally in a rangeof 10 to 99%, preferably 10 to 60%.

When heat is used to fix the ink upon the surface of the membrane,temperatures of about 80° C. up to less than the decompositiontemperature of the membrane are preferred. Pressure may be carried outby manual presses, flat plate presses, a roller or rollers pressingagainst aflat plate backup member or a roller or rollers pressingagainst a backup roller or rollers or by any suitable means of applyingpressure, manually or automatically. Elevated temperatures suitable forfixing the particles upon the surface may be achieved by heating themembrane having catalyst ink upon the surface in an oven or othersuitable heating device, by heating a pressure plate or plates, byheating a pressure roll or rollers,by external heat lamps, or by anyother suitable heating devices or combination of the foregoing. Whenpressure and heat are applied simultaneously, the heating device may beincorporated in the pressure device such as the pressure plate or thepressure roller or rollers, or there may be any suitable combination ofexternal sources of heat used in conjunction with pressure devices.

Generally, the length of time for the application of heat is notcritical and is dependent upon the temperature and/or pressure beingapplied to thesurface of the membrane having catalyst particles orpowder deposited thereon. Typically, heat is applied from less thanabout 1 minute to about2 hours, and when a pressure of about 2030 kPa(20 atm) is used with a temperature of about 130° C., heat is appliedfor less than about 1minute to about 15 minutes, preferably about twominutes.

In preferred embodiments, any pressure plate or roller surfaces used tofixthe particles of catalyst materials upon the surfaces of the membranemay have a release surface, such as a coating of TEFLON®, fluorocarbonor other suitable release material thereon.

The electrode structure may also be applied to the surface of themembrane by the so-called decal process. In particular, an alternativeto printing the catalyst layer directly onto the membrane electrolyte isto coat, paint, spray or screen print the catalyst onto a piece ofsubstrate or paper and subsequently transfer the catalyst from thesubstrate or paper to the membrane. A version of this process is wellknown in fuel cell art.In this process the ink formulation is preparedand preferably mixed with water and an amount of TEFLON®, preferablyTEFLON® 30B (commercially available from E. I. du Pont de Nemours andCompany) is added. TEFLON® should constitute 10% to 70%, preferably 30%to 50% of the catalyst layer dry weight. The mixture is flocced usingheat or by acidification. The mixture is cast onto a piece of paper by avacuum filtration. The water is withdrawn through the paper leaving thesolid, flocced filtrate in a uniform layer on the paper. This paper isthen placed, catalyst side down, on a piece of teflonated or wetproofedcarbon paper. The carbon paper, catalyst layer and catalyst-layer paperbacking are sandwiched between sheets of filter paper and the excesswater is pressed out. The assembly is removed from the press and thefilter paper is discarded. The paper is now sprayed lightly with watercausing the paper fibers to swell. The paper can now be removed and whatremains is a TEFLON®-bonded, diffusion-type fuel cell electrode. Theelectrodes aregenerally dried and sintered at about 332° C. for about 15to 30 minutes.

It is also possible to print the electrode onto a paper backing asdescribed in the prior art. After the ink is dried, two such printedpapers are placed on either side of a fluorinated ion exchange membranewhich is preferably in the unhydrolyzed form, typically the sulfonylfluoride form. The papers are placed so that the printed areas areplaced facing the membrane. The membrane usually being transparent andthe paper being somewhat translucent, permits easy registry of the twoprinted catalyst layers. The sandwich so formed is placed between theheated platens of a press. The press is closed and raised to a pressureof about 1380 kPa (200 psi) at the surface of the membrane and to atemperature of about 127° C. This condition is maintained for about 2minutes after which the membrane and electrode structure package iswithdrawn. To remove the paper from the membrane and electrodestructure, water may be sprayed on the paper which causes the fibers toswell. The paper can now be peeled from the catalyst layer which is nowfirmly bonded to the membrane.

The advantage of the decal approach is that it permits the removal ofmost ink solvents prior to pressing. These processes have also yieldedlayers which are less subject to mudcracking. The approach simplifiesfixturing the membrane for printing. It also permits printing andstorage of large quantities of catalyst layer, which also facilitatesthe production of customized membrane and electrode structures.

The membrane on which the electrode layer is formed is not limiting. Itcanbe made of a polymer having ion exchange groups such as carboxylicacid groups, sulfonic acid groups, phosphoric acid groups and phenolichydroxy groups. Suitable polymers include copolymers of a vinyl monomersuch as tetrafluoroethylene and chlorotrifluoroethylene and aperfluorovinyl monomer having an ion-exchange group such as sulfonicacid group, carboxylic acid group and phosphoric acid group or areactive group which can be converted into the ion-exchange group. It isalso possible to use amembrane of a polymer of trifluoroethylene inwhich ion-exchange groups such as sulfonic acid group are introduced ora polymer of styrene-divinylbenzene in which sulfonic acid groups areintroduced.

The ion exchange membrane is preferably made of a fluorinated polymer.The term "fluorinated polymer" generally means a polymer in which, afterloss of any R group by hydrolysis to ion exchange form, the number of Fatoms is at least 90% of the total number of F, H and Cl atoms in thepolymer. For chloralkali cells, perfluorinated polymers are preferred,through the R in any --COOR group need not be fluorinated because it islost during hydrolysis. The fluorinated polymers are preferablyso-called carboxyl polymers or so-called sulfonyl polymers.

The carboxyl polymers have a fluorocarbon backbone chain to which areattached the functional groups or pendant side chains which in turncarry the functional groups. When the polymer is in melt-fabricableform, the pendant side chains can contain, for example --[--CFZ--]_(t)--W groups wherein Z is F or CF₃, t is 1 to 12, and W is --COOR or --CN,whereinR is lower alkyl. Preferably, the functional group in the sidechains of the polymer will be present in terminal O--[--CFZ--]--_(t) --Wgroups wherein t is 1 to 3.

Polymers containing --(OCF₂ CF{CF₃ })mOCF₂ CF{CF₃ }CN side chains, inwhich m is 0, 1, 2, 3, or 4, are disclosed in U.S. Pat. No. 3,852,326.Polymers may contain --(CF₂ CFZ)_(m) OCF₂ COOR side chains, where Z andR have the meaning defined above and m is 0, 1, or 2 (preferably 1).

Polymers containing terminal --O(CF₂)_(v) W groups, where W is definedas --COOR or --CN and v is from 2 to 12 are preferred. These groups maybe part of --(OCF₂ CFY)_(m) --O--(CF₂)_(v) --W side chains, where Y=F,CF₃ or CF₂ Cl. Especially preferred are polymers containing such sidechains where v is 2, and where v is 3. Amongthese polymers, those withm=1 and Y=CF₃ are most preferred. The abovereferences also describe howto make these fluorinated ion exchange polymers.

The fluorinated polymer may also be so-called sulfonyl polymers. Thesulfonyl polymers with which the present invention is concerned arefluorinated polymers with side chains containing the group --CF₂ CFR_(f)SO₂ X, wherein R_(f) is F, Cl, CF₂ Cl or a C₁ to C₁₀ perfluoroalkylradical, and X is F or Cl, preferably F. Ordinarily, the side chainswill contain --OCF₂ CF₂ CF₂ SO₂ X or --OCF₂ CF₂ SO₂ F groups, preferablythe latter. For use in chloralkali membranes, perfluorinated polymersare preferred. Polymers containing the side chain --OCF₂ CF{CF₃ }O)_(k)--(CF₂)j--SO₂ F, where k is 0 or 1 and j is 3, 4, or 5, may be used.Polymers may contain the side chain --CF₂ CF₂ SO₂ X where X is F or Cl,preferably F. The above references also describe how to make thesefluorinated ion exchange polymers.

Preferred polymers contain the side chain --(OCF₂ CFY)_(r) --OCF₂CFR_(f) SO₂ X, where R_(f), Y and X are defined aboveand r is 1, 2, or3. Especially preferred are copolymers containing the side chain --OCF₂CF{CF₃ }OCF₂ CF₂ SO₂ F.

Polymerization can be carried out by the methods known in the art.Especially useful is solution polymerization using ClF₂ CFCl₂ solventand (CF₃ CF₂ COO)₂ initiator. Polymerization can also be carried out byaqueous granular polymerization, or aqueous dispersion polymerizationfollowed by coagulation.

The perfluoro ion exchange polymer is a copolymer of tetrafluoroethylenewith one of the functional comonomers disclosed herein. The ratio oftetrafluoroethylene to functional comonomers on a mole basis is 1.5 to5.6:1. For each comonomer, the most preferred ratio oftetrafluoroethyleneto functional comonomers is determined by experiment.Copolymers with high ratios of tetrafluoroethylene to comonomers areless soluble than those with low ratios. It is desirable to have aliquid composition with most micelles of less than 100 Angstroms, but analternative is to remove the larger micelles by filtration orcentrifugation.

The polymer of the ion exchange membrane may also be formed fromcopolymersof monomer I with monomer II (as defined below). Optionally, athird type of monomer may be copolymerized with I and II.

The first type of monomer is represented by the general formula:

    CF.sub.2 ═CZZ'                                         (I)

where:

Z and Z' are independently selected from the group consisting of --H,--Cl, --F, or --CF₃.

The second type of monomer consists of one or more monomers selectedfrom compounds represented by the general formula:

    Y--(CF.sub.2).sub.a --(CFR.sub.f).sub.b --(CFR.sub.f).sub.c --O--[CF(CF.sub.2 X)--CF.sub.2 --O].sub.n --CF═CF.sub.2(II)

where

Y is selected from the group consisting of --SO₂ Z, --CN, --COZ, andC(R³ f)(R⁴ f)OH;

Z is --I, --Br, --Cl, --F, --OR, or --NR₁ R₂ ;

R is a brached or linear alkyl radical having from 1 to about 10 carabonatoms or an aryl

R³ f and R⁴ f are independently selected from the group consisting ofperfluoroalkyl radicals having from 1 to about 10 carbon atoms;

R₁ and R₂ are independently selected from the group consisting of --H, abranched or linear alkyl radical having from 1 to about 10 carbon atomsor an aryl radical;

a is 0-6;

b is 0-6;

c is 0 or 1;

provided a+b+c is not equal to 0;

X is --Cl, --Br, --F, or mixtures thereof when n>1;

n is 0 to 6; and

R_(f) and R_(f) are independently selected from the group consisting of--F, --Cl, perfluoroalkyl radicals having from 1 to about 10 carbonatoms and fluorochloroalkyl radicals having from 1 to about 10 carbonatoms.

Particularly preferred is when Y is --SO₂ F or --COOCH₃ ; n is 0 or 1;R_(f) and R_(f) are --F; X is --Cl or --F; and a+b+c is 2 or 3.

The third, and optional, monomer suitable is one or more monomersselected from the compounds represented by the general formula:

    Y'--(CF.sub.2).sub.a' --(CFR.sub.f).sub.b' --(CFR.sub.f).sub.c' --O--[CF(CF.sub.2 X')--CF.sub.2 --O].sub.n' --CF═CF.sub.2(III)

where:

Y' is --F, --Cl or --Br;

a' and b' are independently 0-3;

c is 0or 1;

provided a'+b'+c' is not equal to 0;

n' is 0-6;

R_(f) and R_(f) are independently selected from the group consisting of--Br, --Cl, --F, perfluoroalkyl radicals having from about 1 to about 10carbon atoms, and chloroperfluoroalkyl radicals having from 1 to about10 carbon atoms; and

X' is --F, --Cl, --Br, or mixtures thereof when n'>1.

Non-ionic (thermoplastic) forms of perfluorinated polymers described inthefollowing patents are also suitable for use in the present inventionbecause they are easily softened by heating and make it easy to bond themembrane to the electrode. Membranes which are suitable are described inthe following U.S. Pat. Nos.: 3,282,875; 3,909,378; 4,025,405;4,065,366; 4,116,888; 4,123,336; 4,126,588; 4,151,052; 4,176,215;4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333; 4,270,996;4,329,435; 4,330,654; 4,337,137; 4,337,211; 4,340,680; 4,357,218;4,358,412; 4,358,545; 4,417,969; 4,462,877; 4,470,889; and 4,478,695;European PatentApplication 0,027,009. Such polymers usually haveequivalent weight in the range of from about 500 to about 2000.

The copolymerization of the fluorinated olefin monomer and a monomerhavingsulfonic acid group or a functional group which is convertibleinto sulfonic acid group, if necessary, the other monomer can be carriedout bymethods known in the art. The polymerization can be carried out,if necessary, using a solvent such as halohydrocarbons by a catalyticpolymerization, a thermal polymerization or a radiation-inducedpolymerization. A fabrication of the ion exchange membrane from theresulting copolymer is not critical, for example it can be known methodssuch as a press-molding method, a roll-molding method, anextrusion-molding method, a solution spreading method, adispersion-molding method and a powder-molding method.

The thickness of the membrane is typically 25 to 175 microns, especially25to 125 microns.

A preferred example of a commercial sulfonated perfluorocarbon membraneis sold by E. I. du Pont de Nemours and Company under the tradedesignation NAFION®. The sulfonic groups are chemically bound to theperfluorocarbon backbone, and prior to operation the membrane ishydrated to yield a membrane having at least about 25% water based upondry weight of membrane.

In the case of anion exchange resins the ionic group is basic in natureandmay comprise amine groups, quaternary ammonium hydroxides, theguanidine group, and other nitrogen-containing basic groups. In bothcases, that is,in those where the ionic groups are acidic groups or inthose cases where the ionic groups are basic, the ionizable group isattached to a polymericcompound, typical examples of which are aphenolformaldehyde resin, a polystyrene-divinyl-benzene copolymer, aurea-formaldehyde resin, a melamine-formaldehyde resin, and the like.

Hydrolysis of the functional groups of the ion exchange membrane may becarried out by any number of methods known in the art. Hydrolysis mayoccur before or after applying the ink to the surface of the membrane,preferably after. The membrane may be hydrolyzed (i.e., converted to itsionic form) by reacting it with, in the case of --SO₂ F pendant groups,25 wt. % NaOH under the following conditions: (1) immerse the filminabout 25 wt. % sodium hydroxide for about 16 hours at a temperature ofabout 90° C.; and (2) rinse the film twice in deionized water heated toabout 90° C., using about 30 to about 60 minutes per rinse.

The membrane can be reinforced by supporting said copolymer on a fabricsuch as a woven fabric or a net, a nonwoven fabric or a porous film madeof said polymer or wires, a net or a perforated plate made of a metal.

The membrane and electrode structure may be stored in any convenientmanner. Preferably, the membrane and electrode is pressed between asheet of paper such as filter paper and stored in an airtight plasticbag.

The membrane and electrode structure is particularly useful in a fuelcell.As it is well known, fuel cells are devices capable of generatingelectricity by electrochemically combining an oxidizable reactant,termed a fuel, and a reducible reactant, termed an oxidant. Thereactants are fluids, either liquids or gases, often hydrogen andoxygen, and usually fed continuously to the cell from separate externalsources. The fuel cellis divided into compartments by the membrane andelectrode structure.

Each electrode is electronically conductive, adsorbs the fuel or oxidantemployed, presents an active material for the electrode reaction, anddoesnot oxidize unduly under the operating conditions of the cell. Whenfuel and oxidant are concurrently and separately supplied to thedifferent electrodes of the fuel cell, an electrical potential willdevelop across the electrodes. When an electrical load is providedacross the electrodes,an electrical current flows therebetween, theelectrical energy thus represented being generated by theelectrocatalytic oxidation of fuel at one electrode and the simultaneouselectrocatalytic reduction of oxidant at the other.

The membrane and electrode structure is also useful in electrolyticcells. In the operation of an electrolytic cell for the preparation ofan alkali metal hydroxide by the electrolysis of an aqueous solution ofan alkali metal chloride in the electrolytic cell, an aqueous solutionof an alkali metal chloride is fed into the anode compartmentpartitioned by the cationexchange membrane and water is fed into thecathode compartment. Sodium chloride is usually used as the alkali metalchloride. It is also possibleto use the other alkali metal chloride suchas potassium chloride and lithium chloride. The corresponding alkalimetal hydroxide can be producedfrom the aqueous solution in highefficiency and a stable condition for a long time. The electrolytic cellusing the ion exchange membrane having the electrode layers can be aunipolar or bipolar type electrolytic cell.

EXAMPLES

Membrane and electrode structures were prepared and tested as follows:

Membrane. The ion exchange membrane is the NAFION® NE 112F membrane(made and sold by E. I. du Pont de Nemours and Company). The membranehas a thickness of 0.05 mm (2 mil) in the unhydrolyzed form. The ionexchange polymer is a sulfonyl fluoride polymer having an equivalentweight of 1100. The membranes are cut into 7.6 cm by 7.6 cm (3 inch by 3inch) sheets.

Fuel Cell. The membranes are tested in a test fuel cell prepared inaccordance with pending patent application Ser. No. 07/824,414, J.Kellandand S. Braun to Analytic Power Corporation. The unitized cell isplaced between the plates of a test cell fixture and the entire testfixture is placed in a single cell test stand. The compressive load onthe cell is about 2760 kPa (400 psi) and is applied in a press. The cellis preconditioned using humidified hydrogen and oxygen reactants. Whenthe preconditioning is complete and the cell is at a temperature ofabout 82° C. (180° F.), and a pressure of about 6.9 kPa (80 psi), thecell is run at varying loads. The reactant utilizations arerelativelylow, less than 20%. The loads are simple resistors which areplaced in series with the cell. A shunt is used to determine the cellcurrent and the voltages are taken from end plate to plate. The cellvoltages reportedinclude electrode polarizations and internal resistivelosses as well as losses in conducting the electricity to the end platesof the test fixture.

Electrode Ink. The electrode ink is prepared in a preweighed bottle witha magnetic stirrer. The following components are added to the bottle:2.6 gm. perfluorinated sulfonic acid NAFION® solution (made from 5% byweight NAFION® polymer, 50% isopropyl alcohol, 25% methanol and 20%water), 390 mg. 1-methoxy, 2-propanol; 2 ml. isopropanol; and 487.9 mg.catalyst (made by Precious Metals Corporation) having 20% Platinum onVULCAN® carbon support. The ink is stirred in the capped bottle for15-30 minutes. The electrode ink is sufficient to prepare approximately10electrodes of about 7 cm by 7 cm (2.75 inches by 2.75 inches).

Membrane and Electrode Fabrication. The precut membrane sheet is placedon a MYLAR® (commercially available from E. I. du Pont de Nemours andCompany) screen with a 7 cm by 7 cm (2.75-inch by 2.75-inch) target. Theelectrode ink is loaded on the screen and pressed through the screenusinga standard hard rubber squeegee. Excess ink is removed from thescreen and returned to the bottle. The membrane is removed from thescreen and warmedunder a lamp. The screen printing process is repeateduntil about 80 mg. ofink is applied to the membrane-typically two tofour applications of ink. The membrane is then inverted on the screen.The foregoing steps are repeated in order to print the electrode ink onthe other surface of the membrane.

The membrane is then placed between two glass-reinforced TEFLON®sheetswhich have been dusted with VULCAN® particles. The composite isthen placed between TEFLON®/Graflex platens. Pressure is applied at 2070kPa (300 psi) (calculated using the entire area of the platen) at 127°C. (260° F.) for two minutes.

The pressed membrane is removed from in between the glass-reinforcedTEFLON® sheets and the membrane and electrode structure is hydrolyzed byimmersing the structure in a solution of 69.0 ml water, 25.0 mlisopropyl alcohol and 6.0 gm. sodium hydroxide (solid solute) for onehour. The membrane and electrode structure is then removed and washedwithlarge amounts of water. The hydrolyzed membrane and electrodestructure is then soaked in 5-10% (by weight) 1-2 Normal H₂ SO₄ aqueoussolution at 75° C. for 15 minutes.

The membrane and electrode structure may be pressed between sheets offilter paper and stored in plastic bags for subsequent use or installedinthe fuel cell. In the fuel cell, the membrane and electrode structureis tested for voltage at varying amps per square foot.

The fuel cell was operated at 37.8° C. and 82.2° C. at 101 kPa (1 atm)and 638 kPa (6.3 atm) using air as the oxidant and at 82.2° C. at 638kPa (6.3 atm) using oxygen as the oxidant. The results are reported inTable 1 below:

                                      TABLE 1                                     __________________________________________________________________________    Temperature = 37.8° C.                                                             Temperature = 82.2° C.                                                             Temperature = 82.2° C.                         Pressure = 101.kPa                                                                        Pressure = 638 kPa                                                                        Pressure = 638 kPa                                    Oxidant = Air                                                                             Oxidant = Air                                                                             Oxidant = Oxygen                                      Amps per ft.sup.2                                                                    Voltage                                                                            Amps per ft.sup.2                                                                    Voltage                                                                            Amps per ft.sup.2                                                                    Voltage                                        __________________________________________________________________________    116    0.766                                                                                0    0.958                                                                                0    0.960                                          168    0.721                                                                                5    0.961                                                                                7    0.987                                          208    0.683                                                                                6    1.007                                                                               10    0.918                                          286    0.623                                                                               15    0.993                                                                               46    0.915                                          361    0.574                                                                               136   0.836                                                                               106   0.897                                          384    0.542                                                                               300   0.824                                                                               149   0.866                                          416    0.520                                                                               354   0.815                                                                               329   0.837                                          476    0.459                                                                               569   0.760                                                                               408   0.776                                          500    0.405                                                                               651   0.744                                                                               551   0.762                                                       754   0.715                                                                               691   0.743                                                       960   0.709                                                                               962   0.743                                                      2063   0.390                                                                              1000   0.726                                                      2406   0.398                                                                              1483   0.455                                                      2636   0.222                                                                              2622   0.414                                                                  3380   0.305                                          __________________________________________________________________________

We claim:
 1. An electrode composition comprising:(a) catalyticallyactive particles; (b) a hydrocarbon selected from the group of1-methoxy-2propanol, 1-ethoxy-2-propanol; 1-methoxy-2-methyl-2-propanol;1-isopropoxy-2-propanol; 1-propoxy-2-propanol; 2-phenoxy-1-propanol;2-ethoxy-1-propanol; 2,3-ethoxy-1-propanol; 2-methoxy- 1-propanol;1-butoxy-2-propanol; or mixtures thereof; and (c) a binder.
 2. Thecomposition of claim 1 in which the hydrocarbon is nonsolid at theprocessing temperatures of the electrode composition.
 3. The compositionof claim 1 wherein the binder is a perfluorinated sulfonyl fluoridepolymer.
 4. The composition of claim 3 in which the polymer ishydrolyzed.
 5. The composition of claim 3 in which the polymer isunhydrolyzed.
 6. The composition of claim 3 in which the polymer is in asolution, suspension or dispersion of alcohol and water.
 7. Thecomposition of claim 1 in which the catalytically active particlecomprises a platinum group metal.
 8. The composition of claim 1 in whichthe catalytically active particle is on a carbon support.
 9. Thecomposition of claim 1 in which the catalytically active particles arepresent at about 5-40 weight percent.
 10. The composition of claim 1 inwhich the hydrocarbon is present at about 50-95 weight percent.
 11. Afuel cell comprising a membrane and electrode structure wherein theelectrode comprises:(a) catalytically active particles; and (b) ahydrocarbon having one or more ether or ketone linkages and an alcoholgroup.
 12. A membrane and electrode structure comprising:(a) an ionexchange membrane having an electrode composition applied thereto, saidelectrode composition comprising:(i) catalytically active particles;(ii) a hydrocarbon having one or more ether or ketone linkages and oneor more alcohol groups; and (iii) a binder.
 13. The composition of claim12 in which the hydrocarbon is nonsolid at the processing temperaturesof the structure.
 14. The composition of claim 12 in which thehydrocarbon is selected from the group of 1-methoxy-2-propanol,1-ethoxy-2-propanol; 1-methoxy-2-methyl-2-propanol;1-isopropxy-2-propanol; 1-propoxy-2-propanol; 2-phenoxy-1-propanol;2-ethoxy-1-propanol; 2,3-ethoxy-1-propanol; 2-methoxy-1-propanol;1-butoxy-2-propanol; or mixtures thereof.
 15. The composition of claim12 in which the binder is a perfluorinated sulfonyl fluoride polymer.16. The composition of claim 15 in which the polymer is hydrolyzed. 17.The composition of claim 15 in which the polymer is unhydrolyzed. 18.The composition of claim 15 in which the polymer is in a solution,suspension or dispersion of alcohol and water.
 19. The composition ofclaim 12 in which the catalytically active particle comprises a platinumgroup metal.
 20. The composition of claim 12 in which the catalyticallyactive particle is on a carbon support.
 21. The composition of claim 12in which the catalytically active particles are present at about 5-40weight percent.
 22. The composition of claim 12 in which the hydrocarbonis present in the electrode composition at about 50-95 weight percent.